ABSTRACT Background: Breast cancer remains a leading cause of cancer-related deaths among women worldwide despite our increasingly broader understanding of the biology, risk factors, and genomic landscape of the disease. Chemoprevention signifies a meaningful strategy for reducing the morbidity and mortality from breast cancer. Feasibility and promise of this approach is illustrated by continued clinical interest in selective estrogen receptor (ER) modulators (e.g., tamoxifen), and more recently, aromatase inhibitors (e.g., exemestane) for chemoprevention of luminal-type breast cancers. Unfortunately, these interventions have side effects and lack activity against ER-negative subtypes of breast cancer (e.g., basal-like breast cancer). Therefore, a safe and inexpensive chemopreventive intervention efficacious against different subtypes of breast cancer is still desirable. The overarching goal of this preclinical research project is to acquire in vivo evidence for chemopreventive efficacy of a highly promising dietary phytochemical (diallyl trisulfide; DATS) from Allium vegetables (e.g., garlic) using rodent models exhibiting significant molecular overlap with basal-like and luminal-type human breast cancers. In vivo evidence of chemopreventive efficacy in human-relevant animal models is a prerequisite for initiation of clinical trials of DATS, which was well-tolerated in a prior first-in-human interventional study with intermittent dosing schedule. Published work, including that from our laboratory, already demonstrates activity of DATS against basal-like and luminal-type human breast cancer cell lines in vitro, and their xenografts and cancer stem cell (bCSC) populations in vivo. Epidemiological studies have also suggested an inverse association between dietary intake of garlic and onions and the risk of breast cancer. The mechanistic aspects of this proposal are exceedingly novel and revolve around a still poorly understood transcription factor, Forkhead box Q1 (FoxQ1). Benefitted by access to the RNA-Seq data from TCGA database and through targeted gene expression profiling, we have identified novel targets of FoxQ1, including Dachshund homolog 1 (DACH1) and monocarboxylate transporter 1 (MCT1). DACH1 is a cell fate determination factor and tumor suppressor, whereas MCT1 has an oncogenic role in regulation of metabolic reprogramming in cancer cells. Our recently published work indicates that FoxQ1 is a direct transcriptional repressor of DACH1, and consequently FoxQ1 expression is inversely associated with that of DACH1 in human breast cancers (TCGA). On the other hand, FoxQ1 expression is positively associated with that of MCT1. We also found that DATS treatment suppresses FoxQ1 and MCT1 expression but induces DACH1 protein in breast cancer cells in vitro. However, the in vivo relevance of these cellular findings is still unclear. Likewise, the contribution of FoxQ1/MCT1 axis to breast cancer progression and its potential role in cancer chemoprevention by DATS are yet to be elucidated. Therefore, the mechanistic studies logically focus on the role of FoxQ1 and its novel downstream targets (DACH1 and MCT1) in breast cancer chemoprevention by DATS. Hypothesis: We hypothesize that oral administration of DATS prevents development of basal-like and luminal-type breast cancer in human-relevant rodent models by suppressing oncogenic processes regulated by FoxQ1 (self-renewal of bCSC and metabolic switch from glycolysis to oxidative phosphorylation). Specific Aims: (1) Determine the in vivo efficacy of DATS for chemoprevention of: (a) basal-like breast cancer using a transgenic mouse model, and (b) luminal-type breast cancer using a rat model; (2) Determine the role of FoxQ1/DACH1 axis in bCSC inhibition by DATS using human basal-like and luminal-type breast cancer cells; and (3) Determine the role of FoxQ1/MCT1 axis in metabolic reprogramming and its inhibition by DATS using cellular models and breast tumor tissue arrays representative of human basal-like and luminal subtypes. The tumor tissues from control and DATS-treated mice and rats (aim 1) will be used to determine the in vivo significance of FoxQ1/DACH1 and FoxQ1/MCT1 axes in breast cancer chemoprevention by DATS. Significance: Preclinical evidence of chemopreventive efficacy is essential to justify clinical trial of DATS in breast cancer patients. The first specific aim directly tests the possibility of chemoprevention of basal-like and luminal-type breast cancer by DATS administration using well-characterized rodent models. The proposed mechanistic studies may distinguish pharmacodynamic biomarker(s) potentially useful in future clinical studies with DATS. Finally, further understanding of the role of FoxQ1 in breast cancer may ultimately lead to novel therapies for this heterogeneous disease.